The Order of Time and Odin's Door - Scott Southall - Bog

Anastasia and Edward Upston are summoned to the Order of Time's London headquarters to provide testimony. Their friend and mentor Dr. Alfred Gregorian has been charged with violating the time-traveling society's rules in relation to their trip to ancient Egypt. The twelve-year-old twins are determined to defend Dr. G and to get a closer look at the inner workings of the Order and its secret academy.
Their trip to London doesn't go according to plan when the twins unexpectedly find themselves stuck a thousand years in the past during the Viking Age. Getting home becomes secondary when they are caught in the midst of a power struggle between Odin and Loki that could instigate the end of the nine realms. Surviving long enough for Dr. G and the Order to rescue them may be the hardest thing they ever do. If they can ....